Method for operating an internal combustion engine especially for motor vehicles

ABSTRACT

The invention relates to a method for operating an internal combustion engine, especially for motor vehicles, the engine having a catalytic converter; in the method, for heating up the catalytic converter, there is a switchover between a homogeneous operating mode with a one-time injection of fuel into a combustion chamber of the engine and an operating mode with subdivided injection of fuel at at least two injection time points into the combustion chamber of the engine; wherein, for subdivided injections, both injection time points lie ahead of an ignition of an air/fuel mixture, the first injection time point essentially corresponds to the injection time point of the homogeneous operating mode during the switchover operation from the homogeneous operating mode to the operating mode with subdivided injection, and the second injection time point of the subdivided injection takes place at first so early that the mixture arising during operation with subdivided injection corresponds approximately to a homogeneous mixture and, after the completed switchover, the second injection time point is shifted toward late until a pregiven mixture preparation is present and, for a switchover from the operating mode with the subdivided injection to the homogeneous operating mode, the shift of the second injection time point takes place in the opposite direction. Furthermore, the invention relates to a computer program which is suitable to carry out the method when the method is carried out on a computer.

RELATED APPLICATION

This application is the national stage of PCT/DE 02/04264, filed Nov.20, 2002, designating the United States.

FIELD OF THE INVENTION

The invention relates to a method for operating an internal combustionengine, especially for motor vehicles. In the method, it is provided forheating a catalytic converter that there is a switchover between ahomogeneous operating mode with a one-time injection and an operatingmode with a subdivided injection of fuel at at least two injection timepoints into a combustion chamber of the internal combustion engine. Inthe subdivided injection, both injection points lie forward of anignition of an air/fuel mixture.

BACKGROUND OF THE INVENTION

A method of this kind is known, for example, from U.S. patentapplication publication U.S. 2004/0055561 A1 wherein a method isdescribed for heating a catalytic converter in internal combustionengines having gasoline-direct injection with the steps:

shifting the ignition to “retard”;

checking whether the charge of the cylinders with air exceeds a pregiventhreshold;

subdividing the fuel injection into two component quantities which areinjected before the ignition when the air charge exceeds the threshold.

Vehicles having internal combustion engines require catalytic convertersin the exhaust-gas system for exhaust-gas purification. These catalyticconverters must be brought to the operating temperature as fast aspossible after a cold start so that means for heating are provided. Forexample, after a cold start, the catalytic converter can be heated viahigh exhaust-gas temperatures. This so-called motoric catalyticconverter heating has the advantage that it can be done withoutadditional components.

In internal combustion engines, the exhaust-gas temperature can, inprinciple, be increased in that the degree of efficiency of combustionis deteriorated. A deterioration of the degree of efficiency of themotoric combustion can, for example, be brought about by a deviation ofthe ignition time point from the optimal time point. The optimal timepoint is defined by the maximum degree of efficiency. With a reductionof the degree of efficiency, the exhaust gas is hotter compared to theoperation without a deterioration in the degree of efficiency.Accordingly, the exhaust gas develops an intensified heating action inthe catalytic converter.

For engines having gasoline-direct injection, there exist, in principle,two possibilities to increase the exhaust-gas temperature without addingadditional components:

1. Retarded ignition to deteriorate the degree of efficiency ofcombustion. The ignited mixture is stoichiometric or slightly lean.

2. Additional injection of fuel after ignition for follow-on combustion.The ignited mixture is very lean (stratified operation).

With the increasing rough running, the retarded ignition is limited fora homogeneous mixture. The emissions can furthermore be improved by aslightly lean exhaust-gas lambda at low catalytic convertertemperatures. A leaning is, however, only possible to a limited extentfor a cold engine.

If a secondary injection is provided for the catalytic converterheating, then the complete combustion of the additional fuel mass has tobe ensured. In order to ensure a reliable and complete combustion in theexhaust manifold, the latter must be optimized in its configuration withrespect to through mixing and low thermal mass. Other targets, such asthe reduction of structural space and the optimization of power canthereby be limited. In principle, the after-reaction takes place to apoorer degree in a cold exhaust manifold. Accordingly, the emissions canhardly be reduced shortly after the start.

Because higher temperatures are present in the combustion chamber, lowemissions can be achieved already shortly after start with an afterburning in the combustion chamber. If the fuel is still to be ignited inthe combustion chamber, then the operating parameters must be heldwithin a narrow window. Especially, the injection must start very earlyand therefore contributes significantly to the torque development. Veryshort injection times are a precondition for small load points whichimplies very high demands on the injection valves.

The mixture preparation changes with the subdivision or splitting up ofthe injection in advance of ignition. With this type of mixture, theengine running can be improved. Basically, for a poorer degree ofefficiency and therefore a retarded ignition time point and a higherexhaust-gas temperature, an improved rough running is achievable and themixture can be more greatly leaned earlier after the start than for ahomogeneous mixture by simple injection. In this way, lower emissionsarise.

However, the accuracy of injection valves at small quantities is verypoor. For this reason, a subdividing of the injection for smaller aircharges is not possible.

In order to ensure a reliable start and run-up of the engine, that is,of the internal combustion engine, a simple homogeneous injection canstill be necessary for this phase. A subdivision of the injection takesplace only when there is a sufficient air charge. In this way, shortinjection times are avoided which would lead to an imprecise fuelmetering.

Because of the subdivision of the injection, a mixture stratificationarises. In this way, a rather rich mixture can be present at the sparkplug while the lambda sum is still lean. A reliable ignition even for alean lambda sum is ensured by the rich mixture about the spark plug.

Notwithstanding late ignition, a reliable rapid ignition of the mixturecan additionally be ensured whereby the quiet running with late ignitionis improved.

A different mixture distribution adjusts with divided injection takingplace in advance of ignition, namely, rich in the center of thecombustion chamber and lean on the wall of the combustion chamber. Forthis reason, the wall heat loss can be reduced. Depending upon thecombustion chamber form and the parameters, the following effects canresult:

-   (i) a higher exhaust-gas temperature with the same exhaust-gas    quantity and therefore more heating power for the catalytic    converter;-   (ii) a low exhaust-gas quantity at the same temperature because the    wall heat losses are lower whereby the dwell times of the toxic    substance components in the exhaust manifold and in the catalytic    converter become longer and an after-reaction is required. The    emissions after the catalytic converter can therefore also be    improved hereby.

Basically, at least once in the start phase, there must be a switchoverfrom the simple homogeneous injection (start and run-up) to the dividedinjection (heat up of the catalytic converter) and back. Since the ratiofor the injection quantity cannot be varied or can be varied onlyslightly, there must be a hard switchover between these two types ofmixtures.

Here, it can happen that the driver perceives the switchover, which caninclude an abrupt change in torque, as a jolt in the motor vehicle. Thetorque development is very different for simple homogeneous injectionand subdivided injection because of different mixture types and thedifferent combustion speeds. For this reason, the ignition time pointmust be abruptly shifted with the switchover and the air charge must berapidly changed. Even when the torque development for this change isprecisely modulated, inaccuracies result because of tolerances ofsensors and actuators, for example, via the imprecise detection of theair charge and the crankshaft angle.

Furthermore, inaccuracies of the fuel injection are also present becausetwo short injection times compared to one long injection time are given.In this way, lambda deviations can additionally occur. This problem canbe eliminated only by more accurate injection valves.

SUMMARY OF THE INVENTION

The invention provides a method wherein a jolt-like change of theadjusting parameters and therefore of the torque can be reduced whilesimultaneously having improved heating of the catalytic converter.

The invention solves this task by a previously described method whereinthe first injection time point essentially corresponds to the injectiontime point of the homogeneous operating mode during the switchoveroperation from the homogeneous mode to the operating mode withsubdivided injection and the second injection time point of thesubdivided injection takes place so early that the mixture arisinghereby corresponds approximately to a homogeneous mixture and, after thecompleted switchover, the second injection time point is shifted toretard until a pregiven mixture preparation is present and, for aswitchover from the operating mode with the subdivided injection to thehomogeneous operating mode, the shift of the second injection time pointreverses, that is, takes place in the direction of the first injectiontime point.

In this way, the mixture preparation can be switched over in such amanner that the torque development of homogeneous and subdividedinjection is still similar. In this way, possible inaccuracies no longerlead to a conceivable torque jump. Only after the switchover thesubdivided injection is changed continuously until the wanted mixturepreparation is achieved.

This shift can take place continuously or step-wise. The individualdiscrete steps can each be so selected that no torque jump is perceivedby the driver.

The optimum here is the continuous displacement.

The task is also solved by a computer program, a control apparatus (openloop and closed loop) as well as an internal combustion engine.

Because the second injection time point lies close to the firstinjection time point directly after the switchover, the mixturecorresponds, shortly after the switchover to an operating mode withsubdivided injection, approximately to a homogeneous mixture havingindividual injection. Since the switchover between simple homogeneousinjection and subdivided injection always takes place with a secondinjection, which lies very early (that is, close to the firstinjection), the ignition time point and the air charge have to beadapted only minimally directly after the switchover. After theswitchover to the subdivided injection, the second injection time pointis then shifted to late, that is, to the actual desired value. In thisway, an adaptation of the air charge quantity takes place and, in theopposite case, an adaptation of the air charge quantity takes place inadvance of the switchback. In this way, as a rule, the air chargequantity is raised for the switchover to an operating mode withsubdivided injection. Furthermore, an adaptation of the ignition timepoint can be necessary.

The invention especially relates to a method wherein, in advance ofswitchover, a check is made as to whether the air charge quantity in thecombustion chamber exceeds a pregiven limit value. This is necessaryinsofar that the accuracy of the fuel metering is reliably given onlystarting with certain injection quantities. With the subdivision of theinjection, the accuracy of the fuel metering is changed because now twoshort injection times are present compared to one long injection time.It is necessary that at least mean air charges are present and both fuelquantities must be approximately of the same magnitude. Only when thelarge air charges are reached, can the first injection quantity bevaried compared to the second injection quantity in the subdividedinjection.

Furthermore, it can be provided that the shift of the second time pointis continuous or takes place in several separate discrete steps. Acontinuous shift with a continuous shift of the ignition time point aswell as of the air quantity is especially preferred.

Furthermore, it can be provided that the ignition time point is shiftedin order to change the degree of efficiency after the switchover fromthe homogeneous operating mode into the operating mode having subdividedinjection and/or before the switchback with the displacement of thesecond injection time point.

Especially for a subdivided injection, a retarded ignition time point ispossible whereby a deteriorated degree of efficiency can be achievedwhich, on the other hand, leads to an improved heating up of thecatalytic converter. Notwithstanding the deteriorated degree ofefficiency because of the retarded ignition time point, the smoothrunning is, however, improved in an operation with subdivided injection.

Finally, the invention includes a computer program which is suitable forexecuting the method described above when it is run on a computer. Thecomputer program can be stored especially on a memory, especially aflash memory.

Furthermore, the invention relates to a control apparatus (open loopand/or closed loop) for operating an internal combustion engine with thecontrol apparatus including a memory on which a computer program asdescribed above is stored. A control apparatus of this kind functionsfor controlling all operations in the engine, for example, the meteringof the particular injection quantities, adjusting the ignition timepoints, metering of the corresponding air quantities, et cetera.

Finally, the invention also includes an internal combustion enginecomprising: a combustion chamber; a fuel injection device via which fuelreaches the combustion chamber; a control apparatus (open loop and/orclosed loop); a catalytic converter wherein, for heating up thecatalytic converter, a switchover is provided between a homogeneousoperating mode with a one-time injection and an operating state withsubdivided injection of fuel at at least two injection time points intoa combustion chamber of the internal combustion engine; for thesubdivided injection, both injection time points lie ahead of anignition of the air/fuel mixture; directly after the switchoveroperation from the homogeneous operating mode to the operating mode withsubdivided injection, the first injection time point correspondsessentially to the first injection time point of the homogeneousoperating state and the second injection time point of the subdividedinjection at first is so close to the first injection time point thatthe arising mixture corresponds approximately to a homogeneous mixtureand the second injection time point then can be displaced toward retardaway from the first injection time point until a pregiven mixturepreparation is present; and, the second injection time point isdisplaceable in the opposite direction for a switchover from theoperating state with subdivided injection to the homogeneous operatingstate.

In total, with the subdivision of the injection, the followingadvantages are achieved because here another type of mixture is present:

the motor running is improved;

a deteriorated degree of efficiency is possible for an improved smoothrunning (retarded ignition time point); and,

the mixture can be more greatly leaned.

At the same time, abrupt changes of the adjusting parameters can beavoided by the described switchover strategy. The mixture preparation isswitched over in such a manner that the torque developments ofhomogeneous injection and subdivided injection are so similar thatinaccuracies (for example, because of tolerances of sensors andactuators) no longer lead to a perceivable abrupt change in torque.Furthermore, no torque losses with switchover are perceivable which thedriver perceives in the form of a jolt.

Further features, application possibilities and advantages of theinvention become apparent from the description of an embodiment of theinvention which follows and which is shown in the figure of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained with reference to the drawingswherein:

FIG. 1 shows an internal combustion engine; and,

FIG. 2 shows traces of parameters in the switchover with a shift of thesecond injection time point and for a spontaneous switchover.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows an internal combustion engine 1 of a motor vehicle whereina piston 2 is movable back and forth in a cylinder 3. The cylinder 3 isprovided with a combustion chamber 4 which, inter alia, is delimited bythe piston 2, an inlet valve 5 and an outlet valve 6. An intake manifold7 is coupled to the inlet valve 5 and an exhaust-gas pipe 8 is coupledto the outlet valve 6. An injection valve 9 and a spark plug 10 projectinto the combustion chamber in the region of the inlet valve 5 and theoutlet valve 6. Fuel can be injected into the combustion chamber 4 viathe injection valve 9. The fuel in the combustion chamber 4 can beignited by the spark plug 10.

A rotatable throttle flap 11 is accommodated in the intake manifold 7via which air is supplied to the intake manifold 7. The quantity of thesupplied air is dependent upon the angular position of the throttle flap11. A catalytic converter 12 is accommodated in the exhaust-gas pipe 8and functions to purify the exhaust gases developed by the combustion ofthe fuel.

An exhaust-gas recirculation pipe 13 leads from the exhaust-gas pipe 8back to the intake manifold 7. An exhaust-gas recirculation valve 14 isaccommodated in the exhaust-gas recirculation pipe 13 with which thequantity of the exhaust gas, which is recirculated into the intakemanifold 7, can be adjusted.

A tank-venting line 16 leads from a fuel tank 15 to the intake manifold7. A tank-venting valve 17 is accommodated in the tank-venting line 16.The quantity of fuel vapor supplied to the intake manifold 7 from thefuel tank 15 can be adjusted with the tank-venting valve 17.

A back and forth movement is imparted to the piston 2 by the combustionof the fuel in the combustion chamber 4. This movement is transmitted toa crankshaft (not shown) and applies a torque thereto.

Input signals 19 are applied to a control apparatus 18 for open loopcontrol and/or closed loop control. The input signals 19 representoperating variables of the internal combustion engine 1 which aremeasured by sensors. For example, the control apparatus 18 is connectedto an air mass sensor, a lambda sensor, an rpm sensor and the like.Furthermore, the control apparatus 18 is connected to an acceleratorpedal sensor which generates a signal which indicates the position of anaccelerator pedal, which is actuable by a driver, and therefore therequested torque.

The control apparatus 18 generates output signals 20 with which theperformance of the internal combustion engine 1 can be influenced viaactuators or positioning devices. For example, the control apparatus 18is connected to the injection valve 9, the spark plug 10 or the throttleflap 11 and the like and generates the signals required to drive thesame.

The control apparatus is, inter alia, provided for controlling (openloop and/or closed loop) the operating variables of the internalcombustion engine 1. Especially, the fuel mass, which is injected by theinjection valve 9 into the combustion chamber 4, is controlled by thecontrol apparatus 18 especially in view to a low fuel consumption and/ora low development of toxic substances. For this purpose, the controlapparatus 18 is provided with a microprocessor (computer) which has aprogram stored in a memory medium (especially in a flash memory) andwhich program is suitable for carrying out the above-mentioned control(open loop and/or closed loop).

The control apparatus 18 especially determines the throttle flap angleand the injection pulsewidth which define essential actuating quantitiesfor realizing the desired torque, the exhaust-gas composition and theexhaust-gas temperature. The actuating variables are to be matched toeach other. A further essential actuating variable for influencing thesequantities is the angular position of the ignition relative to thepiston movement.

In this context, the catalytic converter temperature can be determined.Here, measurements as well as also a modulating from operatingquantities are considered. Especially at the start of the engine, theproblem is, however, present that the catalytic converter 12 does nothave the adequate operating temperature. It is therefore necessary thatthe catalytic converter 12 be brought as rapidly as possible to theoperating temperature after a cold start. This heat-up can take placewith the so-called motoric catalytic converter heating via a highexhaust-gas temperature.

The start of an engine takes place, as a rule, in a first operatingmode, the so-called homogeneous operating mode of the engine 1. Thethrottle flap 11 is partially opened or closed in dependence upon thedesired torque. The fuel is injected into the combustion chamber 4 bythe injection valve 9 during an induction phase caused by the piston 2.With the air, which is inducted simultaneously via the throttle flap 11,the injected fuel is swirled and therewith is essentially uniformlydistributed in the combustion chamber 4. Thereafter, the air/fuelmixture is compressed during the compression phase in order to beignited by the spark plug 10. With the expansion of the ignited fuel,the piston 2 is driven. The occurring torque is essentially dependentupon the position of the throttle flap 11 in the homogeneous operation.The throttle flap is essentially closed in the start phase. With a viewto a low toxic substance development, the air/fuel mixture is adjustedat lambda=1 or is adjusted slightly lean at lambda>1.

To increase the exhaust-gas temperature, it can be provided todeteriorate the degree of efficiency of the combustion in that theignition takes place at a later crankshaft angle. The ignited mixture isadjusted to be stoichiometric or slightly lean. However, for ahomogeneous operating mode, the disadvantage is present that the smoothrunning of the engine is not adequate.

According to the invention, the start of the internal combustion engine1 nonetheless takes place in the homogeneous operating mode because theair quantity during start run-up is not always adequate for a subdividedinjection.

As soon as a conclusion can be drawn as to a sufficiently large or atleast mean air charge from the position of the throttle flap 11 or othersensor signals, a switchover into an operating mode with subdividedinjection is carried out for heating the catalytic converter. Initially,a first injection quantity is injected into the combustion chamber and asecond injection quantity is injected at a later crankshaft angle. Thetwo injection time points lie ahead of the ignition time point of thespark plug 10. With the subdivision of the injection, a mixturestratification arises. A rather rich mixture is present at the sparkplug 10 even though the lambda sum in the total combustion chamber 4 isstill lean. A reliable ignition even for a very lean lambda sum isensured by the rich mixture about the spark plug. Notwithstanding aretarded ignition, a reliable rapid ignition of the mixture isadditionally ensured. In this way, the smooth running increases even forretarded ignition and therewith a deteriorated degree of efficiency. Inthis way, wall heat losses are reduced and higher exhaust-gastemperatures can be achieved for like exhaust gas quantities. In thisway, the heating of the catalytic converter is achieved more rapidly.

With the switchover, the following problems are present: at first, theswitchover can only take place when a minimum air charge is presentbecause, otherwise, the fuel quantity per injection, which is to beinjected, is too low in order to ensure that the control of the fuelmetering has a sufficient accuracy. Furthermore, the torque developmentis very different for simple homogeneous injection and subdividedinjection because of the different mixture types and the differentcombustion speed. Accordingly, with the switchover, the ignition timepoint must be shifted abruptly and the air charge must be changedrapidly. Even when the torque development for these changes can bemodeled precisely, inaccuracies arise because of tolerances of thesensors and actuators. Accordingly, a torque jump can occur which thedriver perceives. Furthermore, lambda deviations can occur because theprecision of the fuel metering is different for the two types ofinjection.

According to the invention, it is therefore suggested that the mixturepreparation is reconfigured in such a manner that the torque developmentof homogeneous injection and subdivided injection is still similar atthe time point of the switchover. Here, a perceptible abrupt change intorque no longer occurs because of possible inaccuracies. After theswitchover, the subdivided injection is continuously changed, that is,the two injection time points are spread apart in that the secondinjection time point is shifted to retard until the desired mixturepreparation is reached.

Directly after the switchover, the second injection time point is soearly that it lies virtually at the first injection time point andtherefore the mixture approximately corresponds to the homogeneousmixture with single injection. The ignition time point and the aircharge must then only be adapted minimally. After the switchover, thesecond injection time point can be shifted continuously to the actualdesired value, that is, in the direction of the ignition time point.Ignition time point and air charge are then adapted to the changedmixture preparation and torque development. Especially, the air chargeis increased in order to counter a torque loss. The ignition time pointcan be shifted further rearwardly when there is uniform smooth runningbecause a divided injection permits a higher smooth running for adeteriorated degree of efficiency than a homogeneous injection.

In total, the mixture types have similar characteristics at theswitchover. Inaccuracies of sensors and actuators therefore do not actdifferently. Especially when the shift of the second injection timepoint of the second fuel quantity takes place slowly and continuously,inaccuracies are not perceptible any longer and they do not lead to areduction in the driving performance of the vehicle.

If the air charge is increased after or during the shift of the secondfuel injection to “late”, the ignition time point is adapted incorrespondence to the higher charge and the changed mixture preparation.Overall, steps and jumps, which result in the maximum torque at optimalignition, can be brought close to each other in such a manner that theindividual operating modes pass approximately continuously one into theother.

FIG. 2 shows in detail how the individual parameters change when theswitchover is changed abruptly or by continuous displacement of thesecond fuel injection with respect to the time point.

FIG. 2 shows, on the left-hand side, the maximum torque for optimalignition, the ignition time point with respect to the distance ahead oftop dead center, the ignition time point with respect to the distance totop dead center and the air charge for a direct switchover to the wanteddesired value for the subdivided injection. The right-hand illustrationsshow, in contrast, a “ramping” in accordance with the invention. Theinjection time point of the second fuel quantity at first lies close tothe first ignition time point and, therefore, the mixture characteristiccorresponds closely to that of the homogeneous injection.

It can be especially well seen that in the lowest diagram, which showsthe maximum torque at optimal ignition for an individual injection overtime, at first a constant torque is present. This torque increasesbecause even at optimal ignition for the subdivided injection, a lowertorque is developed than for a simple homogeneous injection andtherefore, more air charge must be built up already ahead of theswitchover. Therefore, the optimal torque increases for the homogeneousinjection with the air charge. This torque then drops abruptly at thetime point of the switchover which is shown in each case by the centerbroken perpendicular line. This power loss is perceived by the driver asa jolt if the power loss is not adequately compensated via thecorrection of the ignition time point. With the increase of the airquantity as well as the adjustment of an optimal ignition time point,this torque can slowly again be increased.

In a switchover, the second fuel injection is first also early, that is,far forward of the top dead center point and the mixture acts like ahomogeneous mixture at first also after the switchover. With the shiftof the ignition time point and the second injection time point, theoptimal torque, in turn, increases to the end value. A jump is notapproximately to be determined via the switchover.

The second diagram from below shows the spacing from bottom dead centerwith respect to the second injection time point. While it is shown onthe left side that this injection time point is at a relatively shortdistance from top dead center already at the time point of theswitchover, it can be seen in the right-hand diagram that this injectiontime point, at first, shows a considerable distance from top dead centerafter the switchover and then only is moved during the switchover towardthe desired value which lies close to top dead center.

It can likewise be seen that, at first, the ignition time point for ahomogeneous injection has a relatively large distance from top deadcenter, that is, an early ignition takes place because only then is agood smooth running ensured for homogeneous injection. When switchoveris approached, this time point is displaced rearwardly in order for theincreased air charge, which results from the uppermost diagram, tocounter a further increase of the torque for homogeneous operating mode.By shifting the ignition time point rearwardly, the smooth running is,however, deteriorated.

After the switchover, the ignition time point has to be again shiftedabruptly to early in order to first counter a drop in torque for uniformair charge and thereby achieve an optimal ignition. At the later timepoint, the ignition time point can again be shifted rearwardly. A shiftcan take place overall significantly farther rearwardly, that is, theignition can take place at a later time point than in a homogeneous modeof operation because here the smooth running is not affected by thedeteriorated degree of efficiency.

If one now views the right diagram, it can be seen that here too, theignition time points only carry out a small jump because, here too, themixture characteristics are similar at the time point of the switchover.

The air charge is approximately the same for the abrupt switchover aswell as for the continuous switchover because this air charge must beincreased for an operating mode with divided injection in order toobtain an optimal torque.

1. A method for operating an internal combustion engine including aninternal combustion engine for a motor vehicle, the engine having acatalytic converter, the method comprising the steps of: for heating upthe catalytic converter, switching over between a homogeneous operatingmode with one-time injection of fuel into a combustion chamber of saidengine and an operating mode with subdivided injection of fuel at atleast two injection time points into said combustion chamber of saidengine, wherein, for subdivided injections, both injection time pointslie ahead of an ignition of an air/fuel mixture; causing the firstinjection time point to essentially correspond to the injection timepoint of the homogeneous operating mode during the switchover operationfrom the homogeneous operating mode to the operating mode withsubdivided injection; causing the second injection of the subdividedinjection to at first take place so early that the arising mixturecorresponds during operation with subdivided injection approximately toa homogeneous mixture; after the completed switchover, shifting thesecond injection time point toward late until a pregiven mixturepreparation is present; and, for a switchover from the operating modewith the subdivided injection to the homogeneous operating mode,shifting the second injection time point in the opposite direction. 2.The method of claim 1, wherein, in advance of the switchover, a check ismade as to whether the air charge quantity in the combustion chamberexceeds a pregiven limit value. 3.The method of claim 1, wherein, afterthe switchover from homogeneous operating mode into the operating modewith subdivided injection, the air charge quantity is increased when thesecond injection time point is shifted toward late; and, the adaptationof the air charge quantity in advance of the return switching into thehomogeneous operating mode takes place in a correspondingly reversedmanner.
 4. The method of claim 1, wherein, after the switchover from thehomogeneous operating mode into the operating mode having subdividedinjection and/or, in advance of switching back, the ignition time pointis shifted for the shift of the second injection time point.
 5. Themethod of claim 1, wherein the shift of the second injection time pointtakes place continuously or in several discrete steps.
 6. The method ofclaim 1, wherein, during the shift of the second injection to late, theinjected fuel quantity is reduced in order to achieve an overall leanlambda.
 7. The method of claim 6, wherein, with the shift of the secondinjection to early, the injected fuel quantity is again increased inorder to maintain the lean running limit for simple homogeneousinjection.
 8. A computer program, comprising a program suitable forcarrying out a method for operating an internal combustion engine, themethod including the following steps when executing the program on acomputer: for heating up the catalytic converter, switching over betweena homogeneous operating mode with one-time injection of fuel into acombustion chamber of said engine and an operating mode with subdividedinjection of fuel at at least two injection time points into saidcombustion chamber of said engine, wherein, for subdivided injections,both injection time points lie ahead of an ignition of an air/fuelmixture; causing the first injection time point to essentiallycorrespond to the injection time point of the homogeneous operating modeduring the switchover operation from the homogeneous operating mode tothe operating mode with subdivided injection; causing the secondinjection of the subdivided injection to at first take place so earlythat the arising mixture corresponds during operation with subdividedinjection approximately to a homogeneous mixture; after the completedswitchover, shifting the second injection time point toward late until apregiven mixture preparation is present; and, for a switchover from theoperating mode with the subdivided injection to the homogeneousoperating mode, shifting the second injection time point in the oppositedirection.
 9. The computer program of claim 8, wherein the computerprogram is stored in a memory including in a flash memory.
 10. A controlapparatus (open loop and/or closed loop) for operating an internalcombustion engine including an internal combustion engine of a motorvehicle, the control apparatus comprising a memory on which a computerprogram is stored for carrying out the following method steps: forheating up the catalytic converter, switching over between a homogeneousoperating mode with one-time injection of fuel into a combustion chamberof said engine and an operating mode with subdivided injection of fuelat at least two injection time points into said combustion chamber ofsaid engine, wherein, for subdivided injections, both injection timepoints lie ahead of an ignition of an air/fuel mixture; causing thefirst injection time point to essentially correspond to the injectiontime point of the homogeneous operating mode during the switchoveroperation from the homogeneous operating mode to the operating mode withsubdivided injection causing the second injection of the subdividedinjection to at first take place so early that the arising mixturecorresponds during operation with subdivided injection approximately toa homogeneous mixture; after the completed switchover, shifting thesecond injection time point toward late until a pregiven mixturepreparation is present; and, for a switchover from the operating modewith the subdivided injection to the homogeneous operating mode,shifting the second injection time point in the opposite direction. 11.An internal combustion engine comprising: a combustion chamber; a fuelinjection device via which fuel reaches the combustion chamber; acontrol apparatus (open loop and/or closed loop); a catalytic converter;and, for heating up the catalytic converter, said control apparatusincluding means for carrying out the method steps of: switching overbetween a homogeneous operating mode with a one-time injection and anoperating mode with subdivided injection of fuel at at least twoinjection time points into the combustion chamber of the internalcombustion engine; for the subdivided injection, causing both injectiontime points to lie ahead of an ignition of the air/fuel mixture;directly after the switchover operation from the homogeneous operatingmode to the operating mode with subdivided injection, causing the firstinjection time point to essentially correspond to the injection timepoint of the homogeneous operating mode and the second injection timepoint of the subdivided injection at first is so close to the firstinjection time point that the hereby arising mixture correspondsapproximately to a homogeneous mixture and then displacing the secondinjection time point toward late away from the first injection timepoint until a pregiven mixture preparation is present; and, displacingthe second injection time point in the opposite direction for aswitchover from the operating mode with subdivided injection to thehomogeneous operating mode.